Fire-type Magneton glitch (Stadium)
|This glitch is for a home console Pokémon game.
Magnemite and Magneton are the only Pokémon that changed typing between Generations I and II, going from pure Electric to Electric/Steel, where the latter type doesn't exist in the former generation.
If you open up a Generation I game in Pokémon Stadium 2's lab area, and perform some edit that prompts you to save the game there, it will save data based on Stadium 2's interpretation--in particular, any Magnemite or Magneton on the file will have data saved to its structure indicating that its types are Electric (0x17) and Steel (0x09), instead of using 0x17 twice.
When one of these Pokémon is loaded on the Game Boy, viewing its summary will ignore the 0x09 type and display that it is pure Electric, as usual. But if you switch back to the original Pokémon Stadium with that game still in the Transfer Pak, you can attempt to enter such a Magnemite or Magneton into a battle, in which case the summary screen before confirming its entry will try to display icons for its types. The Electric type icon displays without a hitch, but 0x09 is not a type with a defined icon, so it falls through past 0x0A, 0x0B, and so on, and finally finds a valid graphic after falling through to 0x14, Fire, so that's what the game displays as the secondary type.
This glitch is purely cosmetic, as the supposed Fire typing has no effect, and Magnemite and Magneton continue to undergo type interactions as though they were pure Electric.